Vapor-phase formation of alkyl isocyanate-derived self-assembled monolayers on titanium dioxide

Titanium dioxide (TiO2) surfaces were modified at atmospheric pressure by exposure to the vapor of alkyl isocyanates having different alkyl chain lengths (CH3[CH2]nN=C=O, where n = 3, 4, 7, 11, 15, and 17). Dynamic water contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy confirmed that the vapor-phase treatment produced self-assembled monolayers (SAMs) of the isocyanate molecules on the TiO2 surface through a carbamate linkage without any marked changes in surface morphology. The thickness and hydrophobicity of the isocyanate-derived SAMs depend on both the alkyl chain length and process temperature. As a control, an oxidized silicon (SiO2/Si) surface was treated under the same conditions. Water contact angle hysteresis for the SAM-covered TiO2 surfaces is greater than that observed for SAM-covered SiO2/Si surfaces, suggesting that SAMs on TiO2 surfaces are disordered and/or insufficiently cover the surfaces. This leads to their water sensitivity. Desorption kinetics are also strongly affected by the alkyl chain length, and the SAMs of adsorbed molecules with n = 15 and 17 are relatively stable among these six isocyanate-derived SAMs.